The present invention relates to the inner elongated structure of the roll of a paper/board machine or finishing machine, such as the shaft of a deflection-compensated roll or the suction box of a suction roll.
Current production paper machines run at speeds nearing 2000 m/min and machine widths come close to 11 m. The future development trend is to continue increasing these values.
Increasing both will bring about a changeover to dynamic dimensioning in current deflection-compensated rolls unless new ways are invented for manipulating the specific frequency of the roll so as to prevent the critical specific frequency from falling upon the running zone. A deflection-compensated roll comprises a stationary shaft and a shell arranged to rotate around it, the shell being supported on the shaft by loading elements which exert a loading force against the inner surface of the shell to load the shell towards the backing roll forming a nip with the said roll. In the modem, wider deflection-compensated rolls of calenders, it has been necessary to increase dimensioning by as much as four classes as a result of dynamic dimensioning, which incurs considerable additional expenses. The increase in roll mass also causes problems regarding crane capacity, especially in old mills.
In a suction roll, a perforated shell rotates fitted with bearings on thrust shafts. Inside the shell may be a single- or multi-chamber suction box, the apertures of which open—limited by sealing strips—onto the inner surface of the shell for directing the suction at a specific sector of the suction roll. At the ends of the roll are connectors by means of which external negative pressure can be connected to the suction box. While the negative pressure is connected, a vacuum is formed under the paper web through the wire or the felt. The pressure difference formed removes water from the web to the perforations in the shell or holds the web during transfer. The negative pressure in the chambers is determined in accordance with the intended use of the suction roll. A problem with suction rolls is the deflection of the suction box towards the inner surface of the roll shell while negative pressure is connected to the suction box. In this case, external pressure will deflect the suction box in the direction of its suction inlets, whereby the seals of the suction box are pressed more tightly against the inner surface in the central area of the roll shell, thus wearing the seals more in their center than on the edge zone.
In simplified form, the specific frequency of the roll is determined according to the following formula:
      f    i    =                    λ        i        2                    2        ·        π        ·                  L          2                      ·                  (                              E            ·            I                    m                )                    1        /        2            where                λ is the support constant        L is roll width        EI is stiffness, and        and m is mass.        
From this equation for specific frequency can be seen that its characteristics cannot be efficiently manipulated by any means other than by manipulating stiffness, when the mass remains approximately constant. Another way of eliminating the detrimental effects of the vibrations themselves is to provide so high roll-internal damping that specific frequencies will not be a disadvantage. The aim of the present invention is to provide a solution by means of which the above-mentioned problems can essentially be eliminated.
To achieve this aim, the solution relating to the invention for realizing the inner elongated structure of the roll of a paper/board machine or finishing machine is characterized in that the structure is comprised at least partly of composite material, including reinforcing fibers in matrix material.